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Maybe a signals person can help me with this.

I'm looking at building a circuit that can be remotely controlled, using an electric fence as the comms bus or antenna.

I've seen this working in a production unit but I don't have one of the units to disassemble. electric fence remote control

The working unit allows a person at a location anywhere along the fence line to push the remote against the electric line and with a button, turn the fence unit on or off.

My question is what type of communication system is this likely to be using?

  • The fence's electric line is insulated from the posts, so in theory is a direct non-interrupted connection back to the unit.
  • The remote control is not earthed, but the fence unit is earthed as the unit operates as a single-wire earth return.
  • The distance from the unit may be 0-5 miles.
  • The typical fence pulse voltage is 7kV to 9kV once every 1.5 seconds, via capacitor discharge.
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  • \$\begingroup\$ Comms: probably very basic pulse position data. \$\endgroup\$
    – Andy aka
    Commented Mar 10, 2020 at 8:30
  • \$\begingroup\$ A high frequency signal could be injected over this line. It could be capacitively coupled or magnetically, with a current transformer . \$\endgroup\$
    – Marko Buršič
    Commented Mar 10, 2020 at 8:32
  • \$\begingroup\$ You probably missed the ground connection on the instrument or a conductive strip on the back. While holding it, you are the grounding. \$\endgroup\$
    – Marko Buršič
    Commented Mar 10, 2020 at 8:39
  • \$\begingroup\$ @MarkoBuršič if that is the case, then there would have to be a high impedance designed in between the fence contact point and where the user holds the remote. Normally if you provide a ground path for the fence you get a shock. Even with rubber boots on it is not pleasant. \$\endgroup\$
    – user57037
    Commented Mar 10, 2020 at 8:55
  • \$\begingroup\$ @MarkoBuršič the standard fence meters have a probe that can be placed into the ground and the voltage measured, but this one doesn't. The farmers may be insulated from the ground with thick farming rubber boots and gloves. \$\endgroup\$
    – Dan
    Commented Mar 10, 2020 at 19:35

1 Answer 1

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Don't know specifically how this unit works, but I could propose how I might solve the problem!

Hand unit

Put a magnet coil inside the tip of the hand unit. Monitor the coil so you can detect when the high voltage pulse happens.

After you see the pulse, you know you have an open transmit window before the next high voltage pulse. During that pause, transmit a sine wave onto the wire via magnetic coupling through the internal coil. The best frequency will likely be in the 10's or 100's of kilohertz. Bonus points if you add a capacitor to make the whole loop into a giant tank and have the hand unit transmit on the resonant frequency.

Fence driver unit

Back at the fence driver unit, in between the high voltage pulses you would connect the two ends of the loop to a transceiver. This could be a transformer if you want to look for current, or something like an op-amp if you want to look for voltage (current probably more immune to noise?). Use a resonant tone detector to sense if the hand unit is transmitting. With careful design, you can detect the tone at astonishingly low levels.

You can modulate the tone, or use multiple tones, to transmit actual data with checksums to avoid false positives from noise on the wire.

Note that you want lots of serious protection on the ends of the big loop for things like transients and lightning! That is a very big loop so it it possible for some very bold things to happen! That would also be a great loop antenna for communicating with aliens! :)

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  • \$\begingroup\$ This sounds fairly advanced. Any chance of a basic electronics hobbyist learning how to do this casually, or should this task be outsourced? \$\endgroup\$
    – Dan
    Commented Mar 10, 2020 at 20:08
  • \$\begingroup\$ What is your goal here? Do you already have an energizer and are just looking for a way to avoid buying the remote? Or is there some reason the existing commercial systems will not do what you need? \$\endgroup\$
    – bigjosh
    Commented Mar 12, 2020 at 4:01
  • \$\begingroup\$ I'm looking at sporadically sending data non-line of sight; through some valleys. It would be similar to the datagram of a Sigfox or Lora datagram \$\endgroup\$
    – Dan
    Commented Mar 15, 2020 at 4:39
  • \$\begingroup\$ @dann So you want to send data from a handheld unit somewhere along a 5 mile wire loop to a fixed-position energizer connected to that wire? Could you possibly use Sigfox repeaters to cover the whole area? These repeaters are cheap and robust, and this is exactly what they are designed for so maybe an easier solution? \$\endgroup\$
    – bigjosh
    Commented Mar 15, 2020 at 15:56
  • \$\begingroup\$ I've been looking at a Sigfox or Lora repeater solution, but it will be pretty difficult for a lot of the places I am looking to reach. It also requires relatively expensive installations for the use of cheap devices. A repeater for one valley which may have only one device for instance. The unit will probably not be handheld. \$\endgroup\$
    – Dan
    Commented Mar 18, 2020 at 21:33

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